Technical Field
The application relates to the state of polarization (SOP) of light modulation apparatus and a SOP detection apparatus, in particular to a modulation apparatus capable of modifying the SOP of light in three dimensional space to generate a polarization vector vortex field with spatial distribution and a SOP detection apparatus capable of measuring all Stokes parameters at high speed in real time, and a detection method thereof.
Related Art
Electromagnetic waves are transverse waves, and the state of polarization (SOP) of light is one of most important basic attributes of the light. In each optical field, the SOP of light is an important parameter. For example, in a remote sensing technology, a target object can be effectively recognized by detecting the light of specific SOP, in biomedicine photonics, polarization imaging can improve a resolution of the image and provide pathological change information, and in materials science, polarized light can be used for measuring parameters such as thickness and refraction index of a thin film.
The Stokes vector is required to determine all polarization information of a light beam and is a universal concept representing the SOP of the light, which can represent any SOP light, including linearly polarized light, circularly polarized light, elliptically polarized light and partially polarized light.
Measurement of the Stokes vector requires to measure the light intensity of the light passing a number of (more than or equal to 4) different polarization elements. In past dozens of years, various SOP detection apparatuses are invented in the world, aiming to realize accurate and fast measurement. In general, there are two types of SOP detection apparatuses: time-independent measuring apparatuses and simultaneous measuring apparatuses. The original SOP detection apparatus consists of a rotating crystal wave plate and a polarizer. Though it is simple in structure and mature in theory, the rotation of the wave plate or polarizer depends on manpower or a motor, at least four measurements are required, and the fast measurement requirements cannot be met. In addition, the rotation of the wave plate will cause light beam drifting, the whole system is non-static, as a result, the uncertainty of measurement is increased. The other type of SOP detection apparatus uses a liquid crystal wave plate (or electro-optical crystal) to replace a traditional crystal wave plate, it uses modulation voltage to rapidly change the phase retardance, although the measurement time is reduced, certain rapid measurement requirements are still not met. In addition, such SOP detection apparatus usually requires two or more liquid crystal (or electro-optical crystal) modulators, and the cost is higher.
Therefore, the simultaneous SOP detection apparatus is of great significance. The simultaneous measurement polarimeter (SMP) is usually realized by using wave-front division and amplitude division methods. A wave-front division apparatus generally requires a design of at least four independent miniaturized optical elements in the light path, and then corresponding signals of all elements are detected one by one. The amplitude division apparatus uses a beam splitter prism to divide a light path into more than four paths, each path with different path design, and the signal of each path is detected finally. The wave-front division apparatus has high requirements on micromachining process. The amplitude division apparatuses have more elements, usually larger in size and higher in cost. All kinds of apparatuses correspond to different measuring methods and their calibration work is very complex.
Known from above description, the SOP detection apparatuses have problems on synchronism, costs, stability and calibration difficulty more or less. A new SOP detection apparatus is urgently required.
On another aspect, a polarization vector vortex field with spatial distribution has many important uses, including multichannel light information transmission and polarization remote sensing measurement, singular optics research, multichannel polarization lighting and analysis, and single measurement of the optical axis direction of crystal, etc. At present, a two dimensional spatial phase modulation sheet prepared by some processes such as thin film processing exists in the market, but its price is high and has the defects that the modulation aperture is in-continuous and the modulation variety is not rich.